Filter & Filter Cartridge FAQs

Q.  When should a cartridge filter be acid washed?

A.  Acid washing is perhaps the biggest single factor leading to premature filter cartridge failure! For years, service technicians and homeowners have acid washed D.E. grids. However, cartridge elements do not have the protective D.E. powder coating that grids have and can be permanently damaged by the acid.

As a rule of thumb, acid wash only when required, if at all, and never without thoroughly cleaning the element with a cartridge degreaser first.   Here’s a simple test to determine if acid washing is required; After thoroughly cleaning the cartridge with a degreasing agent, flush the cartridge with a garden hose to remove all traces of soap, etc.

Now apply several drops of acid directly to a small area of the filter media. If bubbling occurs, this indicates a build-up of calcium or minerals, and you should proceed with a mild acid soak. If no bubbling is observed, your cartridge most likely does not require acid washing.

Q.  Is there an easy way to know when to clean a cartridge filter?

A.  Cartridge filters work on the clarification process. They utilize the debris they collect as an aid to improve filtration. As a result, cleaning too often does not allow the filter to work at optimum filtration efficiency, while not cleaning often enough will shorten the life of a cartridge element.

Filter Cleaning Tip – When you replace the element with a new one, note the start-up pressure on the tank gauge. This reading is the system’s total pressure in “pounds per square inch” or psi.

Mark this point with a drop of fingernail polish or a grease pen. Then add 8 pounds to the reading and place another dot. You now have the normal operating pressure for your cartridge in psid or pounds per square inch differential. Allow the filter to operate between these two points and clean whenever the gauge reaches the higher pressure mark.

Q.  Some filter cartridges have one hard end cap and one soft end cap. Why is this?

A.  When cartridge manufacturers design a new filter element, several factors must be considered. One of the most critical is the end cap hardness. To determine the exact degree of shore hardness required, we first must look at the element sealing points.

Sealing points both position the element and prevent water by-pass. Historically, single cartridge systems all used cartridges with inside i.d. friction seals with hard end caps. This design provided both strength and pleat support. However, as filters became more sophisticated, so did end cap design.

Sta-Rite’s Posi-Flow element, for example, uses a soft vinyl bottom to both seal the inside and outside edges and lock the cartridge into the filter base, while the top end cap is a hard compound for cartridge strength and integrity.

Hayward’s Star Clear filter uses hard end caps for integrity with a soft gasket to allow a knife edge top seal. Both the new American Predator and Jacuzzi CFR designs rely on a soft top and bottom to achieve an o-ring seal.

These end cap configurations are excellent designs. Only the compounds change to meet the manufacturers’ specifications. This is just one of many reasons why Unicel builds its replacement filter cartridges to OEM design specifications and why Unicel is #1 in filter quality and reliability.

Q.  I saw a new filter cartridge made with yellow material. What kind of filter is this and why are Unicel filter cartridges white?

A.  Unicel and virtually every brand name company in the filter business uses a spunbonded, 100% polyester material called “Reemay” as their filter medium. Reemay, which has FDA compliance, is approved for milk filtration and is white to indicate purity.

Research shows no added benefit for using a yellow filter cartridge other than being a different color than everyone else. The filter cartridge you mentioned uses a less expensive version of the same spunbonded polyester material that other manufacturers use, but it’s yellow instead of white.

Some low-end manufacturers are utilizing this material to reduce costs; however, there is a significant difference in performance.

Q.  How can I tell when a cartridge filter element needs to be replaced?

A.  With no moving parts or electrical switches to fail, cartridge elements do not have a defined termination point. Instead the fine interstices of the media matrix gradually plug up over time.

In a typical hot tub or spa, the culprit that plugs the media is perspiration and body oils combined with soaps, chemicals and very fine particulate.  Assuming the filter has been properly maintained and correctly sized to the pump, determining when the cartridge is exhausted depends primarily on three factors:

1. Shorter cycle time between cleanings
2. Low water flow rate and high differential pressure
3. Catastrophic failure such as a tear in the media or center core collapse

All three are dependent upon proper spa water chemistry and following a routine maintenance schedule. Tub owners should be reminded that filter elements are plastic and should be handled and maintained accordingly.

All of the following will help maximize the life & performance of a filter cartridge:

• Clean the elements per cleaning instructions.
• Never use a stiff brush to scrub the media.
• Maintain spa water chemistry in proper balance.
• Do not allow the differential pressure to go over 8 psi between cleanings.
• Alternate two sets of cartridges when cleaning.

Q.  Sharp Pleat Folds vs. Rounded Pleats; Is There Really a Difference?

A.  Absolutely! One of the first things a filter expert will look for when examining a hot tub or spa element is the pleat fold or “knuckle radius” of the pleat. Why is this important?

If you look carefully, you will notice that some manufacturers’ pleats are very sharp while others are rounded and even appear puffy. With Reemay, or other spunbonded polyester medias, it is much easier to make a rounded fold than a sharp pleat.

Moreover, a rounded fold can mask the filter manufacturer’s inability to make straight pleats or maintain proper pleat spacing.  Unicel takes great care to make sharp pleat folds for the following reasons:

• Ease of Cleaning – Sharp folds increase the spacing between pleats making it easier to flush out loose debris.

Eliminates Bridging – Allows particles to get down into the pleat instead of laying across the top of two or more pleats. This assures maximum dirt holding.

• Full Utilization of the Pleat Surface – Allows debris to build up from the inside out, critical to obtaining the maximum cycle life between cleanings.

• Maximizes Flow, Minimizes Pressure Loss – by decreasing the surface contact between the inner pleat knuckle and center tube.

* The above pleat information pertains to spunbonded polyester filters. Paper based
filters were not considered for this article.

Q.  Why Do Some Filter Cartridges Use Support Bands, but Not Others?

A.  Support bands are almost exclusively used on pool filter cartridges to help prevent pleat flutter during the filtration cycle. The water flow through spa filters is under much less pressure and support bands would do more harm than good on hot tub and spa filter cartridges.

Q.  What is the filter cartridge cleaning procedure on a chlorine or bromine sanitized hot tub or spa?


1. Remove the cartridge from the filter housing following the manufacturer’s instructions.
2. Use a garden hose with a straight flow nozzle to wash down the filter element. Work from the top down, holding the nozzle at a 45 degree angle, and wash all the pleats with emphasis between pleats.
3. Rinse until all dirt and debris is gone.
4. For all spa cartridges and elements used in swimming pools where perspiration, suntan lotions, and other oils are present, soak the element for at least one hour (over night is more effective) in (1) a commercial filter cleaner; or (2) one cup trisodium phosphate (TSP) to five gallons water; or (3) once cup dishwasher detergent to five gallons of water.
5. Rinse the cartridge again to remove oils and cleaning solution.
6. If the filter has a coating of algae, calcium carbonate (residue from calcium hypochlorite), iron, or other minerals, soak the cartridge in a solution of one part muriatic acid to twenty parts water until all bubbling stops. WARNING: Failure to remove all oils and cleaning solution before acid soaking will result in a permanent restriction of water flow and cause premature cartridge failure.
7. Rinse the cartridge clean an reassemble housing.

*WARNING – Always wear rubber gloves and safety glasses when using acid or chlorine. Do not add water to acid. Do not mix chlorine and acid.

Please Note – Unicel does not recommend the use of diatomaceous earth (DE) with cartridge filters. DE particles can become trapped in the body of the filter and shorten cartridge life. If desired, a cellulose fiber/synthetic DE can be used in moderation.